Heart failure is a significant cause of death worldwide with a number of cardiac etiologies including systolic dysfunction due to cardiomyopathy. However, the prognosis of cardiomyopathy is highly variable among patients, due to undiscovered genetic differences in the population. Identification of genetic risk factors that contribute to these variable outcomes has been refractory to nearly all approaches except for the analysis of candidate genes chosen for obvious biological plausibility. Our project will use an unbiased genetic mapping approach in the mouse to identify novel genes that modulate the progression and outcome of cardiomyopathy. In a well-studied mouse model of dilated cardiomyopathy (the Calsequestrin transgenic mouse), we discovered dramatic strain-specific differences in both disease progression and survival. Using a QTL mapping approach in multiple crosses, we have identified seven distinct genetic loci, Hrtfm1-7 (Heart failure modifier), that alter the disease progression. These loci are currently being isolated in congenic lines. Aim 1: We will phenotypically validate the isolated modifier loci in two distinct cardiomyopathic disease contexts;(1) with the original CSQ transgene as a genetic sensitizer and (2) with a surgical sensitizer using pressure overload induced by aortic constriction. The isolated loci that show robust effects in both models will receive priority for further investigation. Aim 2: We will fine-map the most robust of these loci using multiple approaches. We will generate a series of nested congenic lines for each locus and analyze their phenotypic effects in the context of both the genetic and surgical sensitizers. In parallel, we will continue to generate mapping data with new inbred strains to take advantage of ancestral haplotype sharing patterns to delineate the loci. We will also identify genes that show strain-specific differences in heart expression. Aim 3: We will investigate a compelling candidate gene for the Hrtfm2 locus using mouse transgenic and knockout approaches. As other strong candidate genes are identified for the other loci, the most compelling will undergo this same loss-of-function and gain-of-function validation. Our project's long-term goal is to identify candidate heart-failure modifier genes in the mouse for evaluation in the human cardiomyopathy population. Identification and characterization of these genetic modifiers will increase our understanding of the molecular mechanisms underlying the development and progression of cardiomyopathy and heart failure.